Brazilians are dealing with a series of mudslides that have displaced thousands of people and probably killed hundreds after intense rainfall hit the city. We often take for granted here in the United States that the ground we build upon is stable and secure, but that security is the result of decades of practical engineering knowledge and experience having been built up. We use geotextiles to hold soil in place and buttress slopes with concrete walls and other stabilizers to keep things from falling apart. The effective management of stormwater remains one of the great engineering challenges of this century, even in developed countries. And in places hit hard by natural disasters, like Haiti, the need to keep contaminated runoff from reaching and poisoning clean water supplies is extremely important.